Tertiary amines, alone or mixed with other materials, are widely used in many areas. Known applications for tertiary amines are as fungicides, bactericides, surfactants, ganglionic blocking agents, rust inhibitors, anti-oxidants, surface modifiers for textiles, metal complexing agents, viscosity stabilizers, hypergolic fuels, solvents, pH stabilizers, color stabilizers, and as catalysts in a large variety of reactions such as curing of epoxide resins.
The production of tertiary amines by reductive alkylation of secondary amines with aldehydes has been known for at least 50 years. Yields have ranged from several percent to 100 percent. The best alkylations are obtained using formaldehyde as the alkylating agent and platinum as the catalyst. However, lower yields of tertiary amines are obtained when one employs the higher aliphatic aldehydes and/or the secondary amines with branching.
The reaction of secondary amines with ketones is more difficult and is known to give lower yields, especially as the ketone increases in size or if the ketone or secondary amine contains substituents. Thus, dimethyl ketone is generally the preferred ketone as it is the most reactive. As one or more of the methyl groups attached to the carbonyl group are replaced by larger alkyl groups, or by other groups, the yield of tertiary amine decreases. This decrease in yield is more pronounced as the size of the groups attached to the ketonic carbonyl group increases. It is also known that the presence of substituent groups on the carbon atoms alpha to the carbonyl carbon of the ketone or on the carbon atom alpha to the nitrogen atom of the secondary amine will also cause a drop in the yield of tertiary amine. In U.S. Pat. No. 2,388,807 ketones were found to be either inactive or not as reactive as aldehydes in the formation of tertiary amines, either in acid, or alkaline media, even under drastic reaction conditions.
Two side reactions can occur during the reductive alkylation of an amine with an aldehyde or ketone: (1) hydrogenation of the carbonyl group to an alcohol which negates the use of the aldehyde or ketone as an alkylating agent and (2) aldolization of the carbonyl group by the amine which leads to high molecular weight products rather than the desired tertiary amine. Another problem encountered is the difficulty of product isolation due to the proximity of boiling points of many secondary and tertiary amines. Separation of these compounds in order to obtain a pure tertiary amine is extremely difficult and complicated necessitating the use of costly separation procedures.
A method for the production of tertiary amines from secondary amines and aldehydes or ketones, such as the one hereinafter described, which would greatly increase yields of tertiary amine while minimizing the formation of tarry residues and by-products is of great importance.